We don't need to know if Origin is installed before launching the window,
so we can save on startup time by executing the worker after the window
has become visible.
The reason is that `sys.excepthook` is a global attribute which we
have to unset for threads because we show a Qt dialog in it and we
can't do that from threads. Before this change, we used to unset
it in threads, but since it is a global attr, that was unsetting it
for the whole application. We cannot reliably reset it because we
can have multiple threads executing and there will be race conditions.
To fix this situation, `RareAppException` implements a callback to
be patched into `sys.excepthook` which emits a signal to be serviced
by the the `RareAppException` instance in the main thread.
`RareAppException` can be subclassed to implement the
`RareAppException._handler` method for domain specific handling.
The `RareApp` base class instantiates its own `RareAppException`
instance for early basic handling. `RareAppException` is subclassed
into `RareException` and `RareLauncherExcpetion` in `Rare` and `RareLauncher`
respectively to implement the aforemention domain specific handling.
Each of these classes deletes the previous instance and replace it
with their specialized handlers.
